1. Field of the Invention
The present invention relates to a storage system. More particularly, the present invention relates to a method of communicating data between storage apparatus such as RAIDs connected to each other by a network or between a storage apparatus and a host, which are also connected to each other by a network, and relates to the configuration of a storage device for implementing the method.
2. Description of the Related Art
Popularization of IP storages and a rising demand for remote copying of data have resulted in more frequent data transfers between storage apparatus, which are separated from each other by long distances and connected to each other by a network, or between a storage apparatus and a server, which are also separated from each other by long distances and connected to each other by a network. In addition, the amount of transferred data has been increasing year after year so that it becomes necessary to transmit data having a large amount at a high speed with a high degree of reliability.
As an interface between a server and a storage apparatus connected to the server, a Fibre Channel and an SCSI interface are known. The Fibre Channel is adopted as a standard interface, which composes a SAN (Storage Area Network) and is used for transferring data at a high speed. On the other hand, the SCSI interface offers merits of a high data transmission speed, a short transmission delay and an extremely low probability of transmission error generation. Thus, the SCSI interface is used as a popular conventional protocol for storage apparatus. However, the SCSI interface has a problem of limiting applications of the SCSI interface to only communications at short transmission distances.
In recent years, trials have been made to connect storage apparatus to each other by adopting an iSCSI protocol. The iSCSI protocol is a protocol technology for implementing SCSI processing as an interface technology on a TCP/IP, which is a network technology. At the present time, the IETF (the Internet Engineering Task Force) is carrying out work of standardization of the iSCSI protocol. It is expected that there will be many cases in which packets are lost at an IP layer when data is transferred by adoption of the iSCSI protocol between storage apparatus separated from each other by a long distance.
In general, as a countermeasure for tackling the phenomenon of losing packets in communication of data, there are known an ARQ (Automatic Repeat reQuest) technique, i.e., an automatic retransmission request technique, and an FEC (Forward Error Correction) technique. When data is transmitted over a long distance, it is quite within the bounds of possibility that packets of the data are lost. In consequence, when data is communicated by adoption of the ARQ technique, it takes time to retransmit data and, as a result, the data transmission efficiency decreases. In particular, a network delay included in the time it takes to retransmit data in a data transmission over a long distance increases in proportion to the distance. For this reason, the ARQ technique cannot be said to be a desirable countermeasure.
For example, in Japanese Patent Laid-open No. 2001-7785, there is disclosed a technology adopting both the FEC and ARQ techniques. In accordance with this technology, a communication partner is informed of an error control technique suitable for the line quality of the transmission network by adding a unique word to transmitted data and, on the receiver side, the data is decoded in dependence on the unique word. In addition, in Japanese Patent Laid-open No. 2001-168944, there has been proposed a technique for transmitting 2 or more types of data having different attributes, namely, ISO data and ASY data, and, on the receiver side, processing is carried out to receive the data in accordance with the attributes.
However, the above publications neither describe a relation with storage apparatus nor disclose ways to keep up with changes in FEC redundancy.